Stranded winding for high current electric apparatus



Nov/3, 1970 G. E. LEIBIN GER STRANDED WINDING FOR HIGH CURRENT ELECTRICAPPARATUS Filed May 17, 1968 2 Sheets-Sheet 1 INVENTOR GEORGE E E/B/NGERO 3, 1970 e. E. I EIBINGEIR 3,538,473

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0- ATTORNEY United States Patent 3,538,473 STRANDED WINDlNG FOR HIGHCURRENT ELECTRIC APPARATUS George E. Leibinger, Pittsfield, Mass.,assignor to General Electric Company, a corporation of New York FiledMay 17, 1968, Ser. No. 730,169 Int. Cl. H01f 27/28 US. Cl. 336-487 6Claims ABSTRACT OF THE DISCLOSURE This application discloses highcurrent winding for electric induction apparatus in which the strands ofa multistrand conductor are transposed at axially spaced points alongthe winding in any of several known transposition sequences. In order tomore accurately equalize the volts per turn of each strand, the axialspacing between transposition points is such that the median point oftransposition in each half of the winding is closer to the midpoint thanto the adjacent end of the winding.

My invention relates to windings for electric induction apparatus, andparticularly to windings formed of stranded conductor wherein a singlehelically wound conductor of high current carrying capacity is formed ofa plurality of separately insulated conductor strands stacked togetherin radially superposed relation and electrically connected in parallelcircuit relation.

In high current electric windings for power transformers, reactors andthe like, it is a common practice to form the winding conductor of aplurality of strands, usually stacked in radial superposition tomaximize the number of turns in a single cylindrical winding layer. Thestrands of such conductor are usually separately insulated even thoughall strands are connected together at their ends. Such insulation hasthe primary purpose of subdividing the conductor to minimize local eddycurrents resulting from flux traversing the conductor itself. Theinsulation of strands, however, creates another problem in that coilsformed by radially outer strands include more flux than do coils formedby radially inner strands (i.e., with respect to the midpoint betweenthe inner and outer peripheries of the helical conductor). The fluxdifference results in appreciable difference in the number of volts perturn in the radially spaced-apart strands of a single conductor, i.e.,the reactive voltage drop per turn in a reactor or the induced volts perturn in a transformer winding is not the same for each of a plurality ofradially stacked strands. Since the conductor strands are connected inparallel circuit relation at their ends, each pair of strands forms aconductive loop in which circulating current will be set up as a resultof such voltage differences.

To reduce the circulating currents referred above, it is known totranspose the several radially stacked strands of a conductor betweenradially inner and radially outer positions in the stack in such a waythat each conductor occupies a symmetrical succession of inner and outerpositions as it traverses the axial length of the coil.

for example, be a complete reversal of strand positions eitherindividually or in groups.

In prior transpositions of both the foregoing types it has been commonpractice to locate transposition points at equal axial intervals and toprovide such a number of transpositions that each strand occupies asymmetrical succession of radially inner and radially outer positionsfor equal axial distances in each coil layer. While this disposition ofconductor strands appreciably reduces circulating current it does notentirely eliminate such currents.

Accordingly, it is a principal object of my invention to further reducepower loss due to circulating currents in high current electric windingsformed of multistrand conductor.

It is a more particular object of my invention to reduce circulatingcurrent power loss in stranded electric coil conductors which areradially transposed in progressive, or consecutive, sequence. Anotherobject of my invention is to reduce circulatmg current power loss inelectric coil conductors of the transposed strand type by providing anoptimum variation of transposition intervals for transposition sequencesof both the progressive and non-progressive types.

My invention will be more fully understood and its several objects andadvantages further appreciated by referrmg now to the following detailedspecification taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a fragmentary cross-sectional view of an electric transformerhaving primary and secondary windings concentrically wound upon a singlecore leg to illustrate the origin of leakage flux between such windings;

FIG. 2 is a similar fragmentary cross-sectional view of a two-windingtransformer showing in more detail the configuration of the leakage fluxfield;

FIG. 3 is a fragmentary cross-sectional view similar to those of FIGS. 1and 2, but illustrating also a typical stranded winding conductor;

FIGS. 4 and 5 are detailed multiple cross-sectional views illustratingseveral transposition sequences for a multistrand winding such as shownat FIG. 3;

BIG. 6 is a graphical representation showing one typical manner ofvarying transposition intervals in accordance with my invention; and

FIG. 7 is a fragmentary cross-sectional view of a multistrand windingtransposed in accordance with my mventron. I Referring now to thedrawings, I have illustrated my lnvention by way of example inconnection with a transformer of the concentric winding type as commonlyused for large power transformers, such as furnace transformers andothers having low voltage windings of high current-carrying capacity. AtFIG. 1, I have shown in fragmentary cross-sectional view a transformercomprising a magnetizable core including a core leg 10 and a yokeportion 11. The transformer leg 10 of FIG. 1 may, of course, be onewinding leg of a three-phase or other polyphase transformer. Upon thecore leg 10 is wound a helical high voltage primary winding 12 and aconcentric helical secondary winding 13 of high current carryingcapacity. It will be understod by those skilled in the art that whenboth windings are carrying current under load conditions the voltages ofthe windings 12 and 13 are vectorially in substantially opposite phaserelation and the currents in the windings are also in substantiallyopposing phase relation. The current in the primary winding 12 includesas a component thereof the exciting current. On ampere turn basis thisexciting current is the vectorial different between the primary andsecondary currents and has the effect of producing in the core 10, 11 amain flux represented at FIG. 1 by the single flux line Under loadconditions an appreciable part of the flux generated by the current inthe primary winding 12 does not pass through the core yoke 11, butfringes from the core leg 10 to encircle directly the primary andsecondary windings 12 and 13 through a return path including theconductors, the winding insulation and air. This fringing fiux generatedin the primary windings is shown having a common portion 4: in the coreleg .10 and two return paths outside the core leg. One return path pencircles both the primary and the secondary windings, and anotherreturn path is shown returning in the space between the primary andsecondary windings. Similarly, flux established by load current in thesecondary winding 13 has a common portion passing through the core leg10 and two return paths outside the core leg. One of these pathsencircles both the primary and the secondary windings, and the secondpath qb returns in the space between primary and secondary windings.

It will be noted from the directional arrows on the several flux pathsreferred to that the primary and secondary fluxes p and 5 within thecore leg 10 are in opposite directions. It is the dilference between thetotal primary and secondary flux which results in the main magnetizingflux 41, Similarly, the components of stray primary and secondary fluxradially beyond both of the windings 12 and 13 are in oppositedirections and tend to cancel each other. In the space between theprimary and secondary windings, however, the stray flux from both theprimary and secondary windings are in the same direction and reinforceeach other to establish a flux of considerable proportion known asleakage flux and comprising the components and At FIG. 2 I have shownsimilar cross-sectional views of the primary and secondary windings 12and 13, and have illustrated the leakage flux components and in somewhatmore realistic detail while omitting illustration of the mutual strayflux components (p and As illustrated at FIG. 2, the leakage fiuxcomponents do not uniformly include within their loop paths all of theprimary and secondary winding turns, but in fact fringe out appreciablyat the axially remote ends of the windings so that the axial componentof leakage flux at opposite ends of the windings are of considerablyless intensity than the axial component of leakage flux at the axialmidpoint of the windings. In addition it will be evident from FIG. 2that the leakage flux traverses not only the space between the primaryand secondary windings but also passes in part through the conductors ofthe windings themselves, this being shown particularly with respect tothe large cross-section conductors of the secondary winding 13.

At FIG. 3, I have shown a view similar to that of FIG. 2, but in whichthe high current capacity conductor of the low voltage winding 13 isshown composed of a plurality of separately insulated conductor strands:1 to 8. The strands are so disposed that each winding turn is formed oftwo stacks of radially superposed strands (1 to -4 and 5 to 8 at FIG. 3)with the two stacks axially juxtaposed in side-by-side relation. It willbe understood by those skilled in the art that at their axially remoteends the eight separately insulated conductor strands are electricallyconnected together, as illustrated by the lack of insulationtherebetween in the axially endmost turns. In practice, of course,electrical connection of the ends of the conductors is made in the leadsbeyond the end turns of the winding.

At FIG. 4, I have shown by a plurality of successive cross-sectionalviews 4a to 4 inclusive, one sequence in which the eight conductorstrands shown atFIG. 4 may be radially transposed with respect to eachother at selected axially spaced-apart points along the length of thehelical winding 13. Each cross-sectional view at FIG. 4 is of a singlewinding turn, but it will be understood by those skilled in the art thatthese turns, while axially spaced apart, are not ordinarily axiallyadjacent as shown, but

are spaced axially at intervals along the winding. While I have shownonly eight conductor strands for the purpose of illustration, it will beevident that any odd or even number of strands stacked radially ineither one or more stacks comprising a single conductor may be utilized.

The transposition illustrated at FIG. 4 is a so-called progressive type.In the illustrated case the stranded conductor comprises an even numberof strands with four radially superimposed strands in each of twoaxially juxtaposed stacks. FIG. 4a shows the initial position of thestrands as the conductor enters the first turn of the winding 13. FIG.4b illustrates a first transposition point in which the stack of strands1 to 4 has been shifted radially outward by one strand position withrespect to the adjacent stack of strands 5 to 8. FIG. 4a shows a secondtransposition point at which the radially innermost and radiallyoutermost strands 5 and 4, respectively, have been shifted in oppositedirections axially to place each of them in the adjacent stack ofconductors. FIG. 4d shows a succeeding transportation point at which theuppermost stack of conductors has been moved radially inward and backinto radial alignment with the lower stack of strands. It will beobserved that in this threeelement transposition the conductor strands1, 2 and 3 have been moved radially outward by one position, theconductors 6, 7 and 8 have been moved radially inward by one position,and the conductors 4 and 5 have been moved axially in oppositedirections without changing their radial disposition. It will now beevident to those skilled in the art that by the three successivethree-step transpositions shown at FIG. 4 the conductor strand 1 may bemoved to its radially outermost position at FIG. 4j, While each of theother strands is moved in step-bystep progressive manner radially inwardor radially outward until each strand has occupied each of the fouravailable radially displaced strand positions. Similar progressive, orconsecutive, transpositions may be continued until the conductor strand1 has been moved continuously through all strand positions and back toits initial radially innermost and axially uppermost position.

In further connection with the progressive type transpositionillustrated at FIG. 4, it will be understood by those skilled in the artthat whether one complete radial transportion (illustrated at FIGS. 4ato 4 inclusive) is made or whether two or more such transpositions aremade as the conductor traverses the full length of the helical winding13, it is generally the practice to space the transportation points 4a,412, etc. apart by equal axial (or peripheral) distances along thewinding 13. It will also be understood by those skilled in the art thata progressive type transposition may be carried out by transposinggroups of strands radially in consecutive sequence rather than sotransposing each strand individually. Thus, for example, if each stackincluded thirty-two strands rather than four and four radialtransposition points were provided (as points 4b, 4d, 4g and 4j), eachstrand would be moved by eight positions at each such point rather thanby one position as shown, but the transposition would still beprogressive, or consecutive, in that each radial displacement would bein the same direction and by any equal number of positions within theconfines of one complete transposition.

The progressive type transposition illustrated at FIG. 4 and describedabove is illustrated also at page 63 of a book entitled TransformerEngineering by L. F. Bloom et al., published by John Wiley and Sons inits second edition in 1951.

It is understood by those skilled in the art that a high currentcapacity conductor formed of more than two axially juxtaposed stacks ofconductor strands may be transposed progressively in like manner. See,for example, British Pat. 431,617 wherein the strands in three axiallyadjacent stacks are transposed in progressive sequence. If it is desiredto utilize four axially adjacent stacks of strands, each pair of stacksmay be progressively transposed in the manner described in connectionwith FIG. 4 above.

To illustrate the manner in which a single stack radially superposedstrands forming a winding conductor may be transposed in non-progressivesequence, I have illustrated in a succession of cross-sectional views atFIG. 5 two non-progressive types of transposition known in the art asthe standard transposition and the special transportation. At FIGS. 5aand 5b, I have illustrated a so-called standard transportation in whichstrand positions 1, 2, 3 and 4 are completely reversed radially at asingle transposition point between censecutive winding turns. Such atransposition is more fully described at column 3 of Pat. 2,710,380,DeBuda. At FIGS. 50 and 5d, I have illustrated a so-called specialtransposition in which similar reversal of strand position isaccomplished at a single transposition point by transposing thecondoctors in groups rather than individually. More specifically atFIGS. 50 and 5d the strand group 1-2 is reversed in position withrespect to the strand group 3-4. The radial positional reversals whichcharacterize the standard and special transpositions may, of course, becarried out with any desired number of axially juxtaposed stacksconstituting a small conductor.

It is known that a combination of the standard transposition shown atFIGS. 5a and 5b and the special transposition shown at FIGS. 50 and 5dmay be utilized to place each conductor strand in each available radialposition in non-progressive, or non-consecutive, sequence. For example,if a special transposition (FIGS. 50, 5d) is utilized at the midpoint ofthe upper half of a winding, a standard transposition (FIGS. 5a, 5b) isutilized at the winding midpoint and another special transposition isutilized at the midpoint of the lower half of the winding, it is evidentthat each conductor strand occupies each of the four available radialpositions for A the length of the winding.

Transpositions similar to the standard and the special as describedabove and shown at FIG. 5 may also be carried out by transposing theconductors in groups rather than individually, as described for examplein the DeBuda patent identified above. Moreover, it will be understoodthat the transpositions shown at FIG. 5 may be carried out with aconductor formed of two or more radial stacks of strands, each radialstack of such a conductor being transposed as at FIG. 5 independently ofthe other stacks.

As previously described, it is accepted practice in transposingconductor strands in any of the sequences described above to space thetransposition points uniformly along the axis of the coil. For example,utilizing special and standard transpositions it is the practice tolocate the transposition points at the center and the A and points ofthe winding length. In a progressive type transposition it is thepractice to separate the transposition points by equal axial distances.It should noW be noted that a progressive transposition is complete inregard to radial variation of included axial leakage flux for theseveral strand positions when each strand has been located once in eachavailable radial position. Moreover, if a twostack progressivetransposition is carried through a second transposition sequence so thateach strand traverses both of the juxtaposed stacks, the voltageequalization accomplished takes account also of axial variation ofincluded leakage flux resulting from the flux fringing effect describedin connection with FIGS. 2 and 3 above. In all such transpositions,however, uniform spacing of transposition points axially along thewiding assumes that the axial component of leakage flux is of uniformintensity at all points along the winding axis. Such uniform intensitydoes not in fact exist because of the fringing effect.

I have discovered that the circulating currents in a helical electricwinding formed of transposed multistrand conductors may be furtherreduced by spacing the transposition points non-uniformly along theaxial length of the winding. More specifically, I find that byincreasing the distance between transposition points in moving from theaxial midpoint toward the axially remote ends of the Winding, greateruniformity in the volts per turn between adjacent conductor strands maybe attained. For example, in a progressive type transposition with arelatively large number of transposition points, the spacing betweentranspositions is, according to my invention, progressively increasedapproaching each end of the winding, the transposition intervals being aminimum at the axial midpoint of the winding and a maximum at theaxially remote ends thereor. Similarly, in a non-prograssive typetransposition such as that shown in the DeBuda patent referred to above,the transposition points in the opposite halves of the winding arelocated axially somewhat closer to the center of the winding than to theadjacent end. Thus in each case the median position of the transpositionpoints in each half of the winding on opposite sides of the axialmidpoint is closer to the midpoint than to the adjacent end of thewinding.

The degree of non-uniformity or asymmetry in respect to axialpositioning of transposition points in each axial half of the Windingwill, of course, vary in accordance with the configuration of theleakage field in the particular winding under consideration. Where thefringing of the leakage field is very slight, the inward displacement ofv the median transposition point will be slight, but where the fringingeffect is greater, the inward displacement of such median point will begreater.

From the foregoing it will be apparent that the terms median position oftransposition points and median transposition point have the samemeaning. Such median position or point is that axial position at eachside of the axial midpoint of the winding at which occurs the centralone (or only one) of an odd number of transpostions on that side of thewinding midpoint; or in the case of an even number of transpositions oneach side of the winding midpoint it is that axial position at one sideof the winding midpoint which is midway between the central pair oftranspositions on that side of the winding midpoint.

By way of example, I have shown at FIG. 6 a specific graphicalrepresentation of non-uniformly displaced transposition points for awinding having a progressive type transposition.

At FIG. 6, the curve A represents the desired axial spacing oftransposition points in a winding of the kind described at FIG. 4 interms of the ratio of actual to average spacing at each point along thewinding axis. The abscissa of curve A represents turn location alongwlnding axis in percentage of Winding length, the mid point of the curvehorizontally being the midpoint of the Winding and represented by theaxial distance point 50%. The ordinant of the curve A shown at FIG. 6represents transposition point spacing in terms of ratio to an averageor uniform spacing 1.00. The curve represents an optimum progression inthe spacing of transpositron points along a winding having an evennumber of complete progressive type transpositions and characterized bya leakage flux having approximately half the intensity at the axiallyremote ends of the winding that it has at the winding midpoint. Asillustrated, the curve A shows that at the midpoint of the winding thespacing of transposition points is preferably about .95 of an average,or uniform spacing distance, and that such spacing distance is graduallyincreased from the center toward each end of the winding at a verygradual rate to positions approximately A the axial length from each endof the winding. For the last of the axial distance at the axially remoteends, the spacing between transposition points increases sharply untilthe spacing reaches a maximum of approximately double the average axialspacing at each end of the winding.

As a further illustrated of my invention, I have shown at FIG. 7 across-section view (at only one side of the axis) of a multistrandwinding transposed non-progressively in accordance with the invention.At FIG. 7 the axiallly remote ends of the windings are designated OO andthe midpoint as 50%. The quadrature points are designated 25% and 75%. Astandard transposition of a four strand single stack conductor is shownat the midpoint and designated T Near the quadrature points, butslightly displaced toward the midpoint, each axial half of the windinghas a special transposition designated T Thus, as indicated also for aprogressve winding by the diagram of FIG. 6, the winding of FIG. 7 hasthe median transposition point in each axial half asymmetricallydisplaced toward the winding midpoint.

It may be noted at this point that in some transformers voltage taps aretaken out from an axially intermediate region of the high voltagewinding. In such cases the low voltage winding turns radially adjacentthe taps are spread out axially. Such spreading results in a fringingeffect similar to end fringing, and my invention may be utilized tocounteract such an elfect as .well as to counteract fringing of thefield at opposite ends of the winding. Accordingly, I intend that in theappended claims any reference to ends of a winding shall include suchintermediate fringing points of a spread IWinding.

While I have described my invention in connection with a helical windingformed of a single cylindrical layer of winding conductor, it will ofcourse be understood by those skilled in the art that, if desired, thewinding built in accordance with my invention may comprise more than onecylindrical layer and that the spacing of strand transposition pointsmay be made non-uniform and progressively greater approaching each endof the winding in either one or more of such several cylindrical windinglayers.

Moreover, it will be understood that, while I have shown my inventionapplied to the outer of two concentric windings, it is equallyapplicable to the inner winding of such a pair or the intermediatewindings of a group of three or more concentric windings.

Thus while I have described a particular embodiment of my invention byway of illustration, many modifications will occur to those skilled inthe art, and I therefore wish to have it understood that I intend in theappended claims to cover all such modifications as fall within the truespirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In a helical winding for electric induction apparatus, a conductorhelically wound in at least one cylindrical layer of turns extending atconstant pitch along a central axis, said conductor being formed of aplurality of separately insulated strands electrically connectedtogether at their ends and positioned in radially superposed groupsforming at least two axially adjacent radially extending stacks ofstrands in each winding turn, the strands in ad-' jacent pairs of saidstacks being radially transposed at axially spaced points along saidwinding layer in progressive sequence, the number of transpositionpoints along said winding being such that each strand in each said stackoccupies a succession of radially inner and radially outer positions insaid stack in symmetrical sequence throughout the length of saidwinding, the axial spacing of said transposition points beingprogressively greater as said conductor approaches opposite end of saidwinding than is the spacing at the axial midpoint of said winding.

2. A helical winding according to claim 1 wherein the number oftransposition points is such that each said strand successively occupieseach strand position in an adjacent pair of stacks at least once in asingle traverse through the length of said winding.

3. In a helical winding for electric induction apparatus, a conductorhelically wound in at least one cylindrical layer of turns extending atconstant pitch along a central axis, said conductor being formed of aplurality of separately insulated strands electrically connectedtogether at their ends and positioned in parallel side-by-side radiallysuperposed relation forming at least one radial stack of strands in eachWinding turn, the strands in said stack being radially transposed at atleast three axially spaced apart points along the length of saidwinding, the median transposition point in each axially adjacent half ofsaid winding being closer to the axial midpoint of said winding than tothe adjacent end thereof.

4. A winding for electric induction apparatus according to claim 3wherein the strands in each said radial stack of strands are radiallytransposed in progressive positional sequence at axially spaced-apartpoints along said winding layer and the number of transposition pointsis such that each strand of said radial stack occupies every radiallydiscrete strand position at least once in the course of a singletraverse through said winding layer.

5. A winding for electric induction apparatus according to claim 3wherein the strands of said radial stack are transposed radially inprogressive positional sequence at axially spaced-apart points alongsaid winding layers, the number of transposition points being such thateach strand of said radial stack occupies a succession of radially innerand radially outer positions in the stack as said conductor traversesthe length of said winding, the axial spacing between said points oftransposition being a minimum at the axial midpoint of said winding andbeing progressively greater as said conductor approaches axiallyopposite ends of said winding.

6. A winding for electric induction apparatus according to claim 3wherein said conductor consists of a single radially superposed stack ofconductor strands transposed in fully inverse relationship at the axialmidpoint of said winding and having a single point of transposition ineach axial half of said winding positioned closer to said midpoint thanto the adjacent axial end thereof.

References Cited- UNITED STATES PATENTS 1,629,462 5/1927 Palueif336--187 2,249,509 7/ 1941 Welch et al 33=6-187 XR 2,710,380 6/1955DeBuda 336l87 2,829,355 4/ 1958 Eberle 3l36'187 3,371,300 2/1968 Stein336-187 XR FOREIGN PATENTS 654,028 1/ 196-5 Belgium.

756,929 4/ 1953 Germany.

431,617 1935 Great Britain.

THOMAS J. KOZMA, Primary Examiner

